The present invention relates to an exhaust gas purification method and apparatus therefor, and more specifically, to an exhaust gas purification method and apparatus therefor capable of effectively purifying harmful compounds present in an exhaust gas, in particular, hydrocarbon and the like produced in a large amount at the cold start-up of engine.
In an FTP test (LA-4 mode) effected in correspondence with the intensification of an exhaust gas regulation in the United States, about 70 to 80% of hydrocarbon (hereinafter, abbreviated as HC) of the total amount thereof emitted in the test is emitted within 140 seconds (within first mode of Bag 1) at the cold start-up of an engine after engine cranking. During this period, however, since considerable time is needed before a conventional three-way catalyst reaches its operation temperature, most of the HC is exhausted without being purified.
To solve this problem, there is proposed a technique for disposing an electric heated catalyst (hereinafter, referred to as an EHC) and an adsorbent mainly composed of zeolite in an exhaust gas pipe.
A technique using the EHC is disclosed, for example, in U.S. Pat. No. 5,063,029 for a catalyst converter previously filed by the applicant. According to this technique, the EHC is energized by a battery power supply to heat forcibly and steeply a catalyst on a heater or a main catalyst disposed on a downstream side. This disclosed technique is very effective to reduce the emission of HC at the cold start-up of engine.
On the other hand, as a technique using an adsorbent disposed in an exhaust gas pipe, Japanese Patent Application Laid-Open No. 75327/1990 discloses an automobile exhaust gas purification apparatus comprising a catalyst for purifying harmful compounds in an exhaust gas disposed in an exhaust gas system and Y type zeolite or mordenite disposed upstream of the catalyst. Further, Japanese Patent Application Laid-Open No. 135126/1990 discloses an automobile exhaust gas purification apparatus comprising an exhaust gas purification catalyst disposed in an exhaust gas system and an adsorbent disposed up-stream of the catalyst, the adsorbent including at least one kind of catalyst metal carried at a portion of a monolith carrier coated with zeolite.
Further, Japanese Patent Application Laid-Open No. 56247/1990 discloses an exhaust gas purification catalyst containing zeolite which comprises a first catalyst layer mainly composed of zeolite and disposed on a carrier and a second catalyst layer mainly composed of a noble metal catalyst with an oxidation/reduction ability and disposed on the first catalyst layer.
Further, EP485179 proposed by the applicant discloses a technique made by combining an EHC and adsorbent. This technique includes a catalyst converter for purifying exhaust gas by the combination of an adsorbent or adsorbent-catalyst -composition coated on a heater or an adsorbent composed of a high-silica zeolite having a Si/Al ratio of at least 40 and a catalyst carried by the zeolite and main monolith catalyst and honeycomb heater.
Further, there is devised in SAE Paper No. 920847 a bypass system for disposing a valve upstream of an adsorbent as a method of desorbing HC at the stage that a catalyst is sufficiently activated.
However, the catalyst converter disclosed in U.S. Pat. No. 5,063,029 has a problem in that a dedicated battery must be mounted and a large current connection cable is needed, and is thus expensive.
Further, although Japanese Patent Application Laid-Open No. 75327/1990 describes that when an exhaust gas has a temperature of 300.degree. C. or lower, HC is adsorbed by the adsorbent in a trapper, and when the exhaust gas has a temperature exceeding 300.degree. C., the HC is desorbed from the adsorbent and flows into a catalyst converter and is purified by a catalyst. However, a problem arises in that zeolite capable keeping the HC up to 300.degree. C. is not available now and even C.sub.3 H.sub.6 capable of adsorbing HC at a relatively high temperature violently causes the desorption of the HC at 200.degree. C. and a catalyst located on a downstream side cannot reach an operating temperature (usually 300.degree. C.) by the combination of the action of the endothermic heat caused by the desorption and the action of the adsorbent in the trapper serving as a heat sink. Thus, almost all the desorbed HC is exhausted without being purified.
In the apparatus disclosed in Japanese Patent Application Laid-Open No. 135126/1990, it is described that the catalyst carried by the adsorbent is activated even at the stage that HC begins to desorb from the adsorbent and the catalyst on the downstream side is not sufficiently activated. Thus, HC is purified. However, the catalyst is very slightly activated at a temperature of about 170.degree. C. at which the HC begins to desorb, and further since an exhaust gas has a composition usually controlled to the vicinity of a stoichiometric point of an air/fuel ratio=14.6 (in a fuel rich region at the cold start-up of engine) by an O.sub.2 sensor, the exhaust gas is in a substantially rich region (lack of oxygen) when the HC desorbs and causes a decomposition reaction rather than an oxidation reaction. Thus it cannot be expected that the exhaust gas is activated and thus a large amount of unpurified HC is exhausted.
Further, although Japanese Patent Applications Laid-Open Nos. 75327/1990 and 135126/1990 indicate that Y type and mordenite type are preferable as a type of the zeolite mainly constituting the absorbent, these types of zeolite have a poor heat resistance and insufficient durability because the zeolite contains a large amount of Al.sub.2 O.sub.3.
Further, although the catalyst disclosed in Japanese Patent Application Laid-Open No. 56247/1990 intends to serve a three-way catalyst having a high purification ability of HC in particular at the cold start-up of engine and when an air-fuel ratio is in a rich region, a resulting purification ability is not high because the characteristics at the cold start-up of engine are not investigated and a specific operation method is not executed.
Although EP485179 discloses feeding secondary air at a rate of 200 1/min for 50 seconds after engine start-up as operating conditions, this is premised on that an EHC is used. However, the use of the EHC itself does not solve the above problem caused in the system.
Further, although the apparatus employing the bypass system as disclosed in SAE paper No. 920874 operates well as to a purification ability, when it is practically applied, many problems arise in that a system is complex, a heat resistant valve in a piping lacks of reliability, and the like.
Taking the above problems of prior arts into consideration, an object of the present invention is to provide an exhaust gas purification method and apparatus therefor capable of effectively purifying harmful compounds present in an exhaust gas, in particular, HC and the like produced in a large amount at the cold start-up of an engine without the need for a complex system such as an EHC, bypass system and the like.